Method of recovering fatty acid and alkali by the electrolysis of an aqueous solution of an alkali metal salt of a fatty acid



United States Patent METHOD OF RECOVERING FATTY ACID AND ALKALI BY THE ELECTROLYSIS OF AN AQUE- OUS SOLUTION OF AN ALKALI METAL SALT OFAFATTY ACID Shinzo Okada, Kyoto-shi, and Tamotsu Eguchi, Okayamashi, Japan, assignors, by direct and mesne assignments, of three-fourths to Kurashiki Rayon Co., Ltd., Kurashiki-shi, Okayama Prefecture, Japan, a corporation of Japan, and one-fourth to Air Reduction Company, Ingirplgrated, New York, N. Y., a corporation of New No Drawing. Application February 26, 1953, Serial No. 339,174

Claims priority, application Japan March 1, 1952 Claims. (Cl. 204-72) This invention relates to a method of recovering fatty acid and alkali by the electrolysis of an aqueous solution of an alkali metal salt of a lower fatty acid. The object of this invention is to recover fatty acid and alkali in economically good yield from an aqueous solution of alkali metal salts of lower fatty acid, especially to recover acetic acid and sodium by the electrolysis of sodium acetate.

Recently, alkali acetate has become available as a byproduct in various large-scale manufacturing processes. For example, a large quantity of sodium acetate is ob tained as a by-product in the saponification of polyvinyl acetate with caustic soda into polyvinyl alcohol. Here the recovery of acetic acid and alkali from alkali acetate has been seriously considered, and the electrolysis of this salt has been recognized as one of the profitable recovery methods.

However, the research on electrolysis of the salts of fatty acids hitherto has been mostly on the electrolysis conditions and reaction mechanism in connection with the Kolbe reaction to yield ethane or Hofer-Moest reaction to yield methyl alcohol and not on the reaction to yield the fatty acid. For the recovery of fatty acids, the metals of the iron group, especially nickel, have been proposed as for the electrolysis of solutions having a fatty acid salt concentration of 5 to percent, at temperatures of ESQ-95 C. with a current density of 1.5-2.5 amps. per square decimeter of anode surface. Thishigh operating temperature is undesirable for the material of an electrolytic cell and also for the electrolytic operation. Ebonite and cement are usually used for the ma- 'ice the cell-voltage becomes so high as to make the acid separation, too expensive for commercial. operation.

The inventors have made the research principally on the sodium acetate with the object to recover the acid and alkali economically under the electrolytic conditions applicable for an industrial operation. The result shows that when smooth platinum is used as an anode, the Kolbe reaction or Hofer Moest reaction occurs under the industrial electrolytic conditions and the reaction to yield acetic acid is insuflicient as in reported in the literature of the past, if a single substance of the metals of the iron group, for example, iron or nickel, is used as anode, there occur side reactions such as the Kolbe reaction and the Hofer-Moest reaction only to a small extent even under industrial electrolytic conditions, and here a god yield of acetic acid but on the other hand, a strong anodic attack is observed. Accordingly, in case of using nickel as anode, for example, the electrolysis must be operated under the industrially disadvantageous condition of low current density which is not satisfactory for commercial purposes.

Thereupon, the inventors examined the mechanism of the reaction process of yielding acetic acid, Kolbe reaction and Hofer-Moest reaction and reached the conclusion that the anode must have the characteristics of low oxygen over-voltage and a small amount of anodic attack in order to yield acetic acid, and discovered that the acid-proof alloys and oxides of the metals of the iron group are particularly suitable anode material. If these materials are used as anode under conditions suitable for industrial application such as current density of 5-30 amps. per square decimeter of anode surface, concentration of electrolyte of 200-400 g./l. as CHaCOONa, about 10% concentration of obtained acetic acid and cell temperature of 40-80 C., electrolysis can be carried out with a current efliciency of more than and an anodic attack of about 10 mg./amp. hr. Among these acid-proof alloys in which the metals of iron group are a major constituent, many kinds of acid-proof stainless steels can be used. Stainless steel has many varieties according to its composition and if a stainless steel of excellent acid resistance is used, the current eflicieney is 90-95% under the conditions set forth above, the anodic attack is far smaller than for a nickel anode, which is one example of a single substance of the metals of the iron group, and therefore the novel alloy anodes are stable in continuous electrolysis. Ferro-silicon containing about 15% silicon is also superior to unalloyed metals of the iron group. As an oxide of a metal of the iron group, magnetite can be used. Electrolysis results of some examples are cited below.

COMPOSITION OF STAINLESS STEEL \VHICH WAS USED Composition of alloy of C, Cr, Ni, M Ti, Mo, Cu, Fe,

metals of iron group percent peree [1t percent percent percent percent percent percent Anode number:

No. l 0. ()8 17-19 0.08 17-19 0.08 17-19 0.12 17-19 0.08 17-19 terial of an electrolytic cell, where a temperature of more than 80 C. makes the operation difficult. Furthermore, such low current density, for example, is very low if compared with about 30 amps. per square decimeter being used usually in the electrolysis of sodium chloride and therefore results in a very small capacity of the electrolytic cell. Further, if such a low concentration of the fatty acid salt solution as above is used,

When these stainless steels and others are used as anode, electrolysis result obtained under the conditions of current density of about 10 amps. per square decimeter of anode surface, 300 g./l. electrolyte concentration as CHsCOONa, electrolysis ratio of about 50%, acetic acid concentration of about g./l. and electrolytic cell temperature of 50-60" C. is illustrated as follows.

If iron or nickel, that is a single substance of the metals of iron group, is used as anode under the same conditions as above, the anodic attack is 200 mg./amp. hr. for iron and 184 mg./amp. hr. for nickel; therefore, these anodes cannot be-used.

Next, we will describe the electrolysis method and its conditions;

Electrolysis methd.We can use either of the following methods: A method of passing the electrolyte from thecathode compartment-to the anode compartment bythe mercury cell preventing the mixing of anolyte and catholyte by means of a porous diaphragm like asbestos, or a method of passing the electrolyte from an intermediate compartment to the anode compartment and cathode compartment respectively by the diaphragm cell separating the anode, intermediate and cathode compartments by a porous diaphragm like two sheets of asbestos, using iron or nickel as cathode, or a method of passing the electrolyte to the anode compartment and cathode compartment, respectively, by the diaphragm cell, dividing the anode and cathode compartments by a diaphragm of a small permeability likemicro porous unglazed porcelain diaphragm, using iron or nickel as cathode. We consider the mercury cell the best method whether it be a horizontal mercury cell, vertical mercury cell or vertical rotating cathode cell.

Regarding the electrolysis conditions, it was considered necessary heretofore that an extremely low current density, low acetic acid salt concentration and high tempera ture of the electrolysis cell must be employed to yield acetic acid. This invention, however, obtains satisfactory results with the materials mentioned above under the usual conditions of industrial electrolysis, as commonly in the electrolysis of sodium chloride.

Regarding the concentration of the electrolyte and the temperature of the electrolytic cell, the electro-conductivity of an aqueous solution of sodium acetate is comparatively smaller than that of an aqueous solution of sodium chloride; therefore, the concentration and temperature are desired to be as high as possible in order to decrease the cell voltage.

We investigated the effect of the electrolyte concentration and the cell-temperature on side reactions, for instance the Kolbe reaction and Hofer-Mocst reaction, within the range of industrial electrolysis conditions, and we found that there was only a little eflect within the range of concentration of 200-400 g./l. as CHsCOONa and temperatures of 30-80 C. Regarding the temperature, the durability of the lining materials of the electrolytic cell must be considered, and we found that the ebonite lining now generally in use gave no trouble within said temperature range. In view of cell voltage, material of electrolytic cell lining and electrolytic operation, a temperature ranging from 40 to 80 C. is practicable.

Regarding the concentration of the obtained acetic acid and its decomposition degree, the decomposition degree of the anolyte and the concentration of the obtained acetic acid are not parallel because the electrolyte introduced into the cathode compartment can be partly over flowed from the cathode compartment instead of flowing into the anode compartment as seen in the mercury cell. That is, by controlling the amount of over-flowing catholyte, the concentration of acetic acid can be raised, regardless-of the concentration of undecomposed sodium acetate in the anolyte. The electric current efficiency is not much influenced by the concentration of the obtained acetic acid, but the anodic -attack is increased with increasing concentration. l'n consideration of this result and the separation of acetic acid in the anolytefor example, by a solvent extraction method-it is desirable that the operating concentration of the obtained acetic acid is about l00-1l0 g./l. But when there is no overflow of the catholyte, the efficiency of the electric current is remarkably decreased at a decomposition degree ofabout 60%, and thBI'CfOIQ'thC most preferable decomposition degree is less than 60% of the total decomposition degree.

in view of the capacity of theelectrolytic cell (or vessel), the density of the electric current is desired to be as large as possible. When using an anode according to this invention, the side-reaction has little influence, and therefore does not aflect the yield of materials in the range of current densities of 5-30 amps. per square decimeter, but when a high electric current density is used, the bathvoltage rises and the anodic attack increases. Therefore, a current density of about 10 amps. per square decimeter of anode surface is the most preferable. The bath voltage slightly depends on the electrolysis method and its conditions, but its preferred value is about 4.5-5.5 volts within the range of said current density.

The above description relates to sodium acetate. However, this invention is not limited to sodium acetate and is applicable to the electrolysis of other alkali salts-for example, sodium, potassium and lithiumof fatty acids of the formula R-COOH where R is an alkyl group composed of 1-3 carbon atoms.

The following examples illustrate the method of the invention for recovering acetic acid and caustic soda from an aqueous solution of sodium acetate.

Example 1 Using an aqueous solution of 300 g./ 1. of refined sodium acetate as the electrolyte, dividing the anode compartment and cathode compartment by a diaphragm to prevent the mixing of anolyte and catholyte, using the perforated plate of No. 1 stainless steel as anode, and mercury as cathode, under conditions where the cell temperature was 5060 C., the current density is 10 amps. per square decimeter of anode surface so as to make the total decomposition degree of sodium acetate about 25% and the concentration of sodium amalgam about 0.1%, in a vertical rotating cathode mercury cell, the electrolysis was carried out continuously by an anolyte current system so as to flow the electrolyte continuously into the cathode compartment, about 50% of which over-flowed from the cathode compartment and the remainder diffused into the anode compartment through the diaphragm from the cathode compartment. The results were as follows:

Cell voltage5.3 volts Concentration of obtained acetic acid-107.4 g./l. Anodic current efliciency--9l.5%

Anodic attackl4 mg./amp. hr.

Example 2 When the electrolysis was carried out under conditions where the whole electrolyte was passed from the cathode compartment to the anode compartment through the diaphragm and magnetite was used as anode, under the same conditions as described in Example 1, we obtained the following result:

Current voltage5.5 volts Concentration of obtained acetic acid-105.5 g./l. Anodic current efliciency92% Anodic attack-4.7 mg./ amp. hr.

What we claim is:

l. A process for recovering fatty acid and alkali from an alkali metal salt of a fatty acid which comprises electrolyzing an aqueous solution of an alkali metal salt of a fatty acid of the formula of RCOOH, where R is an alkyl group composed of l3 carbon atoms, with an anode consisting of a member of the group consisting of acidproof stainless steel and magnetite, and with the anode compartment separated by diaphragm, under electrolysis conditions of current density of 5-30 amps. per square decimeter of anode surface, temperature of electrolyzing cell of 40-80 C., concentration of alkali metal salt of fatty acid of ZOO-400 grams per liter and a total decomposition degree of less than 60%.

2. A process for recovering fatty acid and alkali from an alkali metal salt of a fatty acid which comprises electrolyzing an aqueous solution of an alkali metal salt of a fatty acid of the formula of R-COOH, where R is an alkyl group composed of 1-3 carbon atoms, with an anode consisting of acid-proof stainless steel, and with the anode compartment separated by diaphragm, under electrolysis conditions of current density of 5-30 amps. per square decimeter of anode surface, temperature of electrolyzing cell of 40-80" C., concentration of alkali metal salt of fatty acid of 200-400 grams per liter and a total decomposition degree of less than 60%.

3. A process for recovering fatty acid and alkali from an alkali metal salt of a fatty acid which comprises e1ectrolyzing an aqueous solution of an alkali metal salt of a fatty acid of the formula of R-COOH, where R is an alkyl group composed of 1-3 carbon atoms, with an anode consisting of magnetite, and with the anode compartment separated by diaphragm, under electrolysis conditions of current density of 5-30 amps. per square decimeter of anode surface, temperature of electrolyzing cell of -80 C., concentration of alkali metal salt of fatty acid of 200-400 grams per liter and a total decomposition degree of less than 4. The process as defined in claim 2 wherein the alkali metal salt of a fatty acid is sodium acetate.

5. The process as defined in claim 3 wherein the alkali metal salt of a fatty acid is sodium acetate.

References Cited in the file of this patent UNITED STATES PATENTS 2,069,403 Cunningham Feb. 2, 1937 2,592,686 Groombridge et al Apr. 15, 1952 OTHER REFERENCES Glasstone et al.: Electrolytic Oxidation and Reduction (1936), pp. 333-4.

Brockman: Electro-organic Chemistry, 1926, pp. 37-38. 

1. A PROCESS FOR RECOVERING FATTY ACID AND ALKALI FROM AN ALKALI METAL SALT OF A FATTY ACID WHICH COMPRISES ELECTROLYZING AN AQUEOUS SOLUTION OF AN ALKALI METAL SALT OF A FATTY ACID OF THE FORMULA OF R-COOH, WHERE R IS AN ALKYL GROUP COMPOSED OFF 1-3 CARBON ATOMS, WITH AN ANODE CONSISTING OF A MEMBER OF THE GROUP CONSISTING OF ACIDPROOF STAINLESS STEEL AND MAGNETITE, AND WITH THE ANODE COMPARTMENT SEPARATED BY DIAPHRAGM, UNDER ELECTROLYSIS CONDITIONS OF CURRENT DENSITY OF 5-30 AMPS. PER SQUARE DECIMETER OF ANODE SURFACE, TEMPERATURE OF ELECTROLYZING CELL OF 40-80* C., CONCENTRATION OF ALKALI METAL SALT OF FATTY ACID OF 200-400 GRAMS PER LITER AND A TOTAL DECOMPOSITION DEGREE OF LESS THAN 60%. 